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Smooth Moves: Today's Surfaces Often Demand Different Finishes

Surface finish has always been an important parameter for gasket and ring sealing. Head gaskets require a surface that is smooth and flat to cold seal and hold a lasting seal. The question is, “How Smooth?”

By Larry Carley

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When multi-layer steel (MLS) head gaskets became commonplace a number of years ago, there was a lot of concern that aftermarket surfacing procedures might not be able to reproduce the mirror-like finish that the vehicle manufacturers said was absolutely necessary to seal MLS head gaskets. The challenge was to duplicate the factory finish using out-dated equipment and methods that may or may not produce the desired results.

Aftermarket equipment suppliers rose to the occasion and introduced a new generation of high speed precision milling machines that could meet or exceed the factory surface finish requirements for original equipment MLS head gaskets. Wet grinding was out and dry milling was in as the new way to surface cylinder blocks and heads.

Wet grinding was capable of producing high quality surface finishes when done properly. But according to Anthony Usher of Rottler Manufacturing, it was a “very messy" process compared to dry milling.

“Grinding requires a certain amount of pressure to cut metal,” says Usher. “Dry milling does not. It just shaves across the surface. If you’re wet grinding a large diesel block, the pressure and cutting action of the stones can change as the grinding head rides over the surface. The metal between the cylinder bores creates more resistance and cuts differently than the areas around the cylinder bores. This can leave a lot of waviness across the surface of the block – as much as .002˝. You won’t get that with dry milling. It will cut the block flat with no high spots.”

Usher says most shops in the U.S. have converted over to dry milling, but wet grinding is still common overseas. “Some shops have converted their old grinding machines for dry milling, but the results are not the same as what you get with equipment that is designed especially for dry milling. The spindle bearings in old grinding machines are not rigid enough to provide the proper support for precision dry milling.”

Skip Anderson of DCM-Tech agreed that dry milling is the only way to surface today’s engines. “Most of the equipment we sell is for industrial applications, so we are using the same industrial technology in our automotive surfacing equipment. We use a ball screw feed mechanism rather than hydraulics because it is quieter, smoother and more consistent. You won’t get feed rate changes with temperature as you can with a hydraulic feed system. We also use industrial precision bearings for the spindles and balance the rotors so our customers can achieve surface finishes that meet their specification. If a customer wants a surface finish as smooth as 5 to 9 Ra (Roughness Average), our equipment can do it.”
Usher and Anderson both said the best surface finishes are achieved by using the correct inserts for the application: PCD (polycrystalline diamond) for milling Aluminum, and CBN (cubic boron nitride) for cast iron. PCD works well on aluminum because aluminum won’t stick to the insert like it can to a CBN insert. In addition, special PCD and CBN inserts with specific edge profiles may be required for milling hard metals such as blocks made of compacted graphite iron (CGI), diesel heads with precombustion chamber cups or spray welded diesel heads.

“The corner of the insert must be prepared properly to cut smoothly,” said Usher. “We have found that a thin layer of CBN or PCD bonded to carbide provides a good combination of surface finish, tool life and cost. We have six or seven different CBN inserts designed for different kinds of milling applications.”

Other equipment suppliers have a somewhat different take on the selection of inserts. Tim Whitley of T&S Machines & Tools said he recommends using CBN for everything. “It delivers great results and works just as well on aluminum as it does on cast iron. The key is the edge preparation on the insert.”

Using the same insert for milling both aluminum and cast iron saves time because you don’t have to switch inserts when going from one metal to the other. It also works well on bimetal aluminum blocks with iron sleeves.

Whitley said Ford sent him some 4.6L heads to see if he could match the factory finish. When he checked the heads, he found the factory finish was 12 Ra. When he finished the heads on his equipment, he said the finish was 8 Ra. “We can deliver any finish specification the OEMs or gasket suppliers require. The rigidity of our machines makes such smooth finishes possible. You can stop and start the machine halfway through a job and not leave a mark on the finish.”

Matt Meyer of RMC Engine Rebuilding Equipment said he also favors using CBN inserts for most milling applications. “We use a specific edge prep on our inserts so they can cut both aluminum and cast iron. We also have special CBN inserts for cutting blocks with hard sleeves and diesel heads that have been spray welded or have precombustion chamber cups. You don’t want aluminum binding to the insert, especially when you are cutting a bimetal surface. It can drag metal across the surface and leave marks. An aluminum oxide coating on an insert is not a good idea because it can bind with aluminum chips and cut unevenly.”

Meyer said his milling equipment can deliver any surface finish that’s required to seal a gasket. “But I think there’s been a conspiracy among the vehicle manufacturers as to the smoothness that’s really necessary. They’ve been telling everybody that you have to have a mirror-like finish otherwise the gasket won’t seal.
That may have been true with the early generation original equipment MLS gaskets, but it’s no longer true with most aftermarket MLS gaskets. The coatings they are now using can handle a more traditional surface finish.”

When Ford introduced the 4.6L modular V8, they specified a factory surface finish is 8 to 12 microinches Ra. By comparison, many Japanese auto makers such as Honda and Mazda were specifying surface finishes in the 8 to 20 Ra range back in the early 1990s for their engines. The MLS gaskets they were using at that time had two to five layers of heat treated steel, each covered with a relatively thin (.001 in.) coating of nitrile rubber or Viton.
Consequently, the gaskets required a very smooth surface finish. By comparison, traditional solid or perforated steel core head gaskets with composition facings or graphite gaskets typically required a finish in the 54 to 113 Ra (60 to 125 RMS) range.

As gasket technology has evolved, surface finish requirements at the OEM level have eased a bit. Most Asian car makers today specify a surface finish of 20 Ra or less, while most domestic vehicle manufacturers say 30 Ra or less is required.